Hydrostatic vehicle control

ABSTRACT

An apparatus for controlling both pump displacement of a variable displacement hydraulic pump and the quantity of fuel delivered to an internal combustion engine for the purpose of delivering a requested hydraulic flow at a highly efficient operating point. The apparatus includes an underspeed control means for reducing pump displacement in response to the operating speed of the engine lugging below a desired operating speed. Additionally, a fuel control means operates to reduce the quantity of fuel delivered to the engine in response to the operating speed of the engine rising above the desired operating speed.

DESCRIPTION

1. Technical Field

This invention relates generally to a control system for a hydrostaticvehicle, and more particularly to an electronic device for controllingengine speed and hydraulic pump displacement in response to loadssubjected on a hydrostatic vehicle.

2. Background Art

In the field of hydrostatic vehicles, for example, excavators, variabledisplacement hydraulic pumps are typically driven by a prime mover,providing hydraulic power to a plurality of work implements as well asto the drive system. Excavators, being extremely versatile machines, areuseful in performing a large number of different and varied tasks (e.g.pipelaying, mass excavation, trenching, logging, etc.), each task havingits own unique hydraulic flow and pressure requirements. For example,during mass excavation, hydraulic power requirements are quite high withbrief periods of reduced need, but in pipelaying, sustained periods oflow flow during waiting are common with sessions of moderate to highflow.

Prior art has shown that a substantial fuel savings can be realized byreducing engine speed to low idle during these sustained periods ofwaiting. While this method does address the most obvious area for fuelsavings, it is silent on the possibility of conserving fuel duringactive times where less than maximum engine speed and pump flow would berequired. For example, U.S. Pat. No. 4,395,199, issued to Izumi et al.on Jul. 26, 1983, discloses an electronic control system for a hydraulicexcavator which controls swash plate inclination on a variabledisplacement pump in response to operator input via a control lever. Inthis way, the system provides the hydraulic flow requested by theoperator, reduces load on the engine during periods of less than maximumpower requirements and, subsequently, reduces fuel consumption. Althoughthe system does save fuel, it will not minimize fuel requirements, dueprimarily to the inefficiencies resulting from operation of thehydraulic pumps at reduced displacement and continued operation of theengine at a single, compromising revolutionary speed. While the operatorcould manually adjust engine speed to maintain pump displacementrelatively high during actual working, it is recognized that operationof an excavator requires the operator use both hands and both feet. Inview of the fact that a majority of excavator operators lack a usefulfifth limb, manual adjustment of engine speed is necessarily given arather low priority.

The present invention is directed to overcoming one or more of theproblems as set forth above.

DISCLOSURE OF THE INVENTION

In accordance with one aspect of the present invention, an apparatus forcontrolling an internal combustion engine having a fuel injection pumpactuator, and at least one variable displacement hydraulic pump with aload sensing means for detecting hydraulic load and adjustinginclination of a swash plate in response to hydraulic flow and loadrequirements. The apparatus includes a first means which detects thedisplacement of the hydraulic pump and delivers a first signalresponsive to the displacement of the hydraulic pump. A second meansdetects the rotational speed of the engine and delivers a second signalresponsive to the rotational speed. A control means receives the firstsignal and delivers a third signal responsive to the magnitude of thefirst signal. An underspeed control means receives the second and thirdsignals, compares the second and third signals, and delivers a fourthsignal in response to the third signal being greater than the secondsignal. A swash plate actuator means receives the fourth signal andcontrols the angle of inclination of the swash plate in response to themagnitude of the fourth signal. A fuel control means receives the secondand third signals, compares the second and third signals, and delivers afifth signal in response to the third signal being less than the secondsignal. A rack actuation means receives the fifth signal and controlsthe supply of fuel to the engine, responsive to the magnitude of thefifth signal.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates in block diagrammatic form a hydrostatic controlsystem, engine, and hydraulic pump arrangement;

FIG. 2 illustrates a load sensing means for controlling swash plateinclination;

FIG. 3 illustrates a fuel injection pump actuator partly in sectionaldetail and partially in block diagrammatic form;

FIG. 4 is a block diagram explanation of an embodiment of the pumpcontrol method;

FIG. 5 is a block diagram explanation of an embodiment of the fuelcontrol method;

FIG. 6 is a detailed block diagram explanation of the engine speedsetting function; and,

FIG. 7 is a diagrammatic view showing one example of the characteristicof desired engine speed with respect to hydraulic pump displacement asdescribed in FIG. 6.

BEST MODE FOR CARRYING OUT THE INVENTION

Referring now to the drawings, wherein a preferred embodiment of thepresent apparatus 10 is shown, FIG. 1 illustrates an electronic controlsystem 12 for a prime mover 14, preferably being an internal combustionengine 16 controlled by a rack 18 of a fuel injection pump 20. The rack18 is positioned by a known electrohydraulic rack actuator means 22under direction from the control system 12. Variable displacementhydraulic pumps 24,26 are driven by the engine 16 while ahydromechanical load sensing apparatus 28 (shown in greater detail inFIG. 2 and discussed later in this specification), controls inclinationof the swash plates 30,32 in response to detected hydraulic load. Thecontrol system 12 can be divided into three major components: fuelcontrol means 34, underspeed control means 36, and control means 38.

The control means 38 receives first signals from a first means 39 overlines 40,42 responsive to the displacements of each of the hydraulicpumps 24,26, computes a desired engine speed responsive to the firstsignal of greatest magnitude, and delivers a third signal via a line 44representative of the desired engine speed to both the fuel controlmeans 34 and the underspeed control means 36. A second means 46 detectsthe actual rotational speed of the engine 16 and delivers a secondsignal indicative of the actual engine speed to both the fuel andunderspeed control means 34,36. The fuel control means 34 receives thesecond and third signals representing actual and desired engine speed,respectively, compares the two signals, and delivers a fifth signal inresponse to the desired engine speed being less than actual enginespeed. A rack actuator means 22 receives the fifth signal and controlsthe supply of fuel to the engine 16 responsive to the magnitude of thefifth signal. Similarly, the underspeed control means 36 receives thesecond and third signals, compares the two signals, and delivers afourth signal in response to the desired engine speed being greater thanthe actual engine speed. A swash plate actuator means 50 receives thefourth signal and controls the angle of inclination of the swash plateresponsive to the magnitude of the fourth signal. More simply stated,when actual engine speed "lugs" below desired engine the underspeedcontrol means 36 acts to reduce pump displacement and allow the enginespeed to increase under lower load constraints. Should the actual enginespeed rise above desired speed, the fuel control means will reduce thesupply of fuel to the engine allowing the engine to slow to a moreefficient operating point.

FIG. 2 illustrates one embodiment of a hydromechanical load sensingapparatus 28. The apparatus 28 includes the hydraulic pump 24 withpositionable swash plate 30, a plurality of operator actuated valves52,54 for respectively controlling hydraulic fluid flow to a pluralityof work implements 56,58, a flow priority control valve 60, and a ballresolver valve 62 for delivering a load pressure signal of greatestmagnitude to the swash plate actuator 64. The flow priority controlvalve 60 operates to give the implement 56 priority of hydraulic fluidflow over the implement 58. Fully actuating the valve 52 causes thecontrol valve 60 to be biased in a direction where all hydraulic flow isdirected to the implement 56. Alternatively, not actuating the valve 52results in a pressure signal biasing the flow control valve 60 in theopposite direction and thereby directs flow to the valve 54. Varyingdegrees of operation of the valve 52 result in an appropriate quantityof flow being delivered to the implement 56 with the remaining flowavailable to the implement 58. The ball resolver valve 62 receives loadpressure signals from each of the valves 52,54 corresponding to the loadapplied to the implement cylinders. The signal of greatest magnitude ispassed to the swash plate actuator 64 where the position of the swashplate 30 is set corresponding to the magnitude of the signal. A pumpdischarge pressure signal is also delivered via line 65 to the swashplate actuator 64 for maintaining pump output pressure at, for example,a 300 psi differential above that requested by the load pressure signal.

Electronic control of the load sensing apparatus 28 is obtained by theuse of a pilot supply 66, proportional pressure valve 68, and a solenoid70. The proportional valve 68 controls the pressure of the pilot supply66 delivered to the swash plate actuator 64. Operation of the solenoid70 under direction from the underspeed control means 36 regulates theproportional pressure valve 68 controls the pressure delivered to theswash plate actuator 64, and consequently effects the swash plate'sposition.

For example, during operation of the excavator, assume desired enginespeed equals actual engine speed; therefore, the underspeed controlmeans 36 will take no action to alter swash plate position. Shouldactual engine speed drop below desired engine speed, the load sensingapparatus will continue to increase pump displacement to provide therequested flow; however, the underspeed control means 36 will act toreduce pump displacement by actuating the solenoid 70 and providing anunderspeed pressure signal to the swash plate actuator 64. The magnitudeof the underspeed pressure signal is varied by the underspeed controlmeans 36 as a function of the difference between desired and actualengine speed (discussed in greater detail later in this text).

FIG. 3 shows an electrohydraulic rack actuator means 22 for controllablypositioning a rack 18 of a fuel injection pump 20. As is conventional,the fuel injection pump 20 includes a fuel injection pump housing 70 anda reciprocating fuel rack 18 axially movable in opposite fuel-increasingand fuel-decreasing directions (shown in FIG. 3 as being to the left andto the right, respectively).

The actuator means 22 further includes a rack control member 72 which isalso movable in opposite fuel-increasing the fuel-decreasing directions.In the particular system illustrated herein, the rack control member 72is in the form of an annular sleeve or collar. A hydraulic servo system74 is provided to function as a means for moving the fuel rack 18 in itsfuel-increasing and fuel-decreasing directions in response tocorresponding movements of the rack control member 72 and with a forcegreater than that required to move the rack control member 72. Thehydraulic servo system 74 particularly illustrated herein includes acylinder 76, a piston 78, a sleeve 80, and a pilot valve spool 82.

The cylinder 76 is secured to the fuel injection pump housing 70 and hasa passage 84 communicating with the interior of the pump housing 70through which pressurized engine lubricating oil may flow. The piston78, which is ported and stepped and connected to the fuel rack 18 foraxial movement therewith, is disposed for axial movement in the cylinder76. The diameter of the left end 86 of the piston 78 is less than thediameter of the right end 88 of the piston 78 which slides in the sleeve80 fixed within the cylinder 76, and both such diameters are less thanthat of the intermediate piston head 90. The left end 86 of the piston78, the piston head 90, and the cylinder 76 define an annular chamber92. The piston head 90 has an annular surface 94 on the right sidethereof.

The pilot valve spool 82 is mounted within the piston 78 for limitedaxial movement relative thereto, and has a reduced diameter recess 96 incontinuous communication with piston ports 98. The axial length of therecess 96 is sized relative to the piston ports 100 and 102 such thatthe recess 96 does not communicate with either of the piston ports 100and 102 when the pilot valve spool 82 is in the balanced position ofFIG. 3, but will communicate with the piston ports 100 or 102 when movedto the right or the left, respectively, relative to the piston 78.

The rack control member 72 is mounted for limited axial sliding movementon the left end stem 104 of the pilot valve spool 82. The rack controlmember 72 is biased towards the right by a spring 106 which shouldersagainst a spring retainer 108, with rightward movement of the rackcontrol member 72 being limited by a retainer clip 110 fixed to thepilot valve spool stem 104. The rack control member 72 has a pair ofradially extending flanges 112 on one side thereof to provide a pair ofoppositely facing shoulders 114 and 116.

An electrically energizable brushless direct current torque motor 118 ismounted in fixed relation to the cylinder 76 of the servo system 74, themotor 118 has a rotatable rotor 120 movable in opposite fuel-increasingand fuel-decreasing directions. It is a functional characteristic ofsuch a torque motor 118 that its rotor 120 will turn freely in itsbearings when no electrical current is supplied to the motor 118. Whenelectrical current is applied, the rotor 120 will exert a preselectedtorque in one direction, the degree of torque being proportional to theamount of the current applied. In the preferred embodiment, appliedcurrent is controlled by regulating the duration of the applied signalas detailed later in this text.

A coupling means 122 is provided for connecting the rotor 120 of thetorque motor 118 to the rack control member 72 to move the rack controlmember 72 in one of its fuel-increasing or fuel-decreasing directions inresponse to movement of the rotor 120 in its correspondingfuel-increasing or fuel-decreasing direction. In the particular systemshown herein, the coupling means 122 comprises a control lever 124 fixedto the rotor 120 and having a free end 126 confined between theshoulders 114,116 of the rack control member 72.

In the system illustrated in FIG. 3, the torque motor 118 is arranged sothat current applied thereto will cause a torque to be exerted on thecontrol lever 124, urging it to move in a clockwise, fuel-increasingdirection, in turn urging the rack control member 72 in its leftward,fuel-increasing direction. A bias means 128 is provided for biasing therack control member 72 in a direction opposite to the direction that thecoupling means 122 will move the rack control member 72 when the torquemotor 118 is energized. In the particular rack actuator means 22 shownin FIG. 3, the bias means 128 comprises a low rate compression spring130 confined between a fixed spring seat 132 and an extension 134 of thecontrol lever 124. With this arrangement, the spring 130 biases controllever 124 in its fuel-decreasing direction, with the free end 126 ofcontrol lever 124 acting on the shoulder 114 of the rack control member72 to bias such rack control member 72 for movement in itsfuel-decreasing direction.

Operation of the underspeed control means is illustrated in blockdiagram form by FIG. 4. One implementation of the underspeed controlmeans 36 is shown as a first proportional plus derivative feedback means136 for controlling the magnitude of the fourth signal and therebycontrol hydraulic pump displacement. The actual engine speed signal isreceived from the second means 46 and delivered to a low pass filter138, eliminating transients associated with individual cylinderignition. The filtered engine speed signal is then delivered to a firstsumming means 140 where it is added to a negative representation of thedesired engine speed signal. The resulting signal is indicative of anerror signal or the difference between the actual and desired enginespeed. The error signal is then multiplied by a first preselectedcoefficient K_(p2) and delivered to a second summing means 142 as theproportional term of the control equation. Simultaneously, the actualengine speed signal is delivered to a second low pass filter 144 andpassed to a negative input of a third summing means 146. A positiveinput of the third summing means 146 receives the unfiltered actualengine speed signal; and resultingly, the third summing means delivers asignal based on the difference between the filtered and unfilteredsignals, and more particularly a signal indicative of rate of change ofengine speed or the derivative of actual engine speed. The derivativesignal is multiplied by a second coefficient K_(D) and delivered to thesecond summing means 142. A first actuator setpoint means 148 delivers aseventh signal of constant magnitude representative of a maximum angleof inclination of the swash plate to the second summing means 142. Thesecond summing means 142 adds the proportional, derivative, and constantsignals and delivers this sum as an eighth signal for controlling themagnitude of the fourth signal. A processing means 150 receives theeighth signal and accesses a preselected memory location indicative ofthe magnitude of the fourth signal. A software table look-up routinedetermines the magnitude of the eighth signal and retrieves a binarynumber from a memory location determined by the magnitude of the eighthsignal. The binary number determines the duration of the fourth signaland; consequently, controls the hydraulic pump displacement. Forexample, retrieving the number 00000000 would result in delivering afourth signal of minimum pulse width and retrieving the number 11111111causes the processing means to deliver a pulse width of maximumduration. Binary numbers of varying magnitude between the two extremesresult in pulse widths of corresponding variable duration. Those skilledin the art of electronic design will recognize that the implementationof the proportional plus derivative feedback control equation, as shownin FIG. 4, could be implemented as a hardware arrangement, a softwareprogram, or a combination of the two. For example, low pass filters arecommonly available hardware circuits and software configurations of lowpass filters are also known in the art. Similarly, summing means can beprovided by either software or hardware.

From the above description, one can see that during operation of theelectronic control system 12 when actual engine speed is above desiredengine speed, the underspeed control means 36 will deliver an eighthsignal calling for a pump displacement greater than the maximum pumpdisplacement. An eighth signal requesting a pump displacement greaterthan maximum will have no additional affect on pump displacement as thepump can supply no more than the maximum. Consequently, the underspeedcontrol means will act to alter the pump displacement only when actualspeed "lugs" below desired speed.

Operation of the fuel control means 34 is illustrated in block diagramform by FIG. 5. One implementation of the fuel control means 34 is shownto be similar to the underspeed control means 36, in that a secondproportional plus derivative feedback means 152 controls the magnitudeof the fifth signal and thereby controls the supply of fuel to theengine.

The actual engine speed signal is received from the second means 46 anddelivered to the negative input of a third low pass filter 154. Thefiltered engine speed signal is then delivered to a fourth summing means156 where it is added to the desired engine speed signal. The resultingsignal is, once again, indicative of an error signal or the differencebetween the actual and desired engine speed, but opposite in sign to thecorresponding error signal in the underspeed control means. The errorsignal is then multiplied by a third preselected coefficient K_(P1) anddelivered to a fifth summing means 158 as the proportional term of thecontrol equation. Simultaneously, the actual engine speed signal isdelivered to a fourth low pass filter 160 and passed to a negative inputof a sixth summing means 162. A positive input of the sixth summingmeans 162 receives the unfiltered actual engine speed signal; andresultingly, the sixth summing means 162 delivers a signal based on thederivative of actual engine speed. The derivative signal is multipliedby a fourth coefficient K_(D1) and delivered to the fifth summing means158. A second actuator setpoint means 164 delivers a tenth signal ofconstant magnitude representative of a maximum allowable rack positionto the fifth summing means 158. The fifth summing means 158 adds theproportional, derivative, and constant signals and delivers this sum asan eleventh signal for controlling the magnitude of the fifth signal. Aprocessing means 166 receives the eleventh signal and accesses apreselected memory location indicative of the magnitude of the fifthsignal. A software look-up routine determines the magnitude of theeleventh signal and retrieves a binary number from a memory locationdetermined by the magnitude of the eleventh signal, as described in theoperation of the underspeed control means 36. The binary numberdetermines the duration of the fifth signal and; consequently, controlsthe rack position and fuel supply.

Operation of the fuel control means 34 is similar to that of theunderspeed control means 36 except for the difference in sign of theproportional term. With the constant term being set to call for maximumrack, only negative proportional terms will have an influence onreducing rack position, or more precisely, the fuel control means willact to reduce the supply of fuel when actual engine speed exceedsdesired engine speed. Unlike the underspeed control means 36, callingfor greater than maximum allowable rack 18 will have the result ofincreasing rack 18 beyond its rated position. To prevent this phenomenonfrom occurring, the additional step of setting the proportional term tozero in response to the actual engine speed signal being less than thedesired engine speed signal has been added to the fuel control means.Checking to see if the error signal is greater than zero and setting theerror to zero if the condition exists is shown as occurring subsequentto multiplying the error signal by the constant K_(P1). Should the errorbe less than zero, then the signal is passed unaltered.

FIG. 6 illustrates the control means 38 in block diagram form and canbest be explained in conjunction with the graphical representation ofengine speed versus pump displacement shown in FIG. 7. As discussedearlier, the control means 38 functions to determine a desired enginespeed based on the hydraulic pump displacement. While the block diagramsof FIG. 6 can be most easily explained as steps in a software routine,those skilled in the art of electronic control design recognize thateither portions, or all of the software may be replaced by hardwarecircuitry without departing from the spirit of the present invention.FIG. 7 shows the desired engine speed signal being controllably settableto one of a plurality of preselected levels in response to the hydraulicpump displacement signal being within one of a plurality ofcorresponding ranges. More particularly, the desired engine speed signalis a first preselected level in response to the pump displacement signalbeing less than a first preselected magnitude for a preselected durationof time. For example, the desired engine speed is set to a standby speedof about 1140 rpm in response to the pump displacement being less than5% for approximately 2 seconds. Further, the desired engine speed signalis a second preselected level in response to said pump displacementsignal being in a range between the first preselected magnitude and asecond preselected magnitude. More precisely, the desired engine speedis set to some operator selected working speed when pump displacement isbetween 5% and 40%. A third range exists where the desired engine speedsignal is directly proportional to the first signal when the firstsignal is greater than the second preselected magnitude. The rampportion of the graph between the working speed and a maximum speedillustrates one possible proportional curve. However, the working speedis adjustable to a plurality of discrete levels, one example being thedashed line of FIG. 7, necessitating the slope of the ramp portion beadjusted to fit between the maximum speed and the new working speed.Maximum desired engine speed is advantageously set to correspond withmaximum pump displacement.

FIG. 6 illustrates one implementation of the graph of FIG. 7. Pumpdisplacement signals are received by the block 200 for each of the pumps24,26 over the lines 40,42. The signal of greatest magnitude is selectedand delivered to the block 202 where the signal is filtered to removetransient displacements which can occur at very low pump displacement.Block 204 receives the filtered signal and sets a variable DESNE5 to oneof two values. The first value corresponds to standby engine speed andwill be assigned to the variable DESNE5 if the pump displacement signalfalls below 5% for a period greater than two seconds. The second valuecorresponds to maximum desired engine speed and will be assigned to thevariable DESNE5 at all times when the pump displacement signal risesabove 5%.

Block 206 also receives the pump displacement signal of greatestmagnitude and uses a software table look-up routine to assign aproportional desired engine speed to the variable DESNE3. The tablelook-up routine accesses a memory location based on the pumpdisplacement and retrieves a desired engine speed stored there. Forexample, the graph of FIG. 7 shows a pump displacement of approximately50% corresponding to a desired engine speed of about 1700 rpm. In thisexample, the table look-up routine would access the memory locationcorresponding to 50% pump displacement, retrieve the desired enginespeed of 1700 rpm, and set the variable DESNE3 to 1700 rpm.

Block 208 is similar in operation to block 206 as it also employs theuse of a table look-up routine to set a desired engine speed variable.The variable DESNE1 is set to a working speed in response to an operatorpositionable thumbwheel switch 210. The block 208 receives a signal fromthe thumbwheel switch 210 indicative of the operator selected level,accesses an appropriate memory location, and assigns the value stored inthat memory location to the variable DESNE1.

Each of the variables DESNE1, DESNE3, DESNE5 are received by block 212where a variable DESNE is first assigned the value contained withinvariable DESNE1. The variables DESNE, DESNE3 are compared and if thevariable DESNE3 is greater than the variable DESNE then the variableDESNE is redefined to be equal to the variable DESNES. More simplystated, the working speed is compared to the proportional speed, if theproportional speed is greater than the working speed then desired enginespeed is set to the proportional speed. The variable DESNE is thencompared to the variable DESNE5 and if the variable DESNE is greaterthan the variable DESNE5 then the variable DESNE is redefined to beequal to the variable DESNE5. This step compares the desired enginespeed, which has been set to either the working or porportional speed,to either the maximum speed or the standby speed, depending upon whichvalue block 204 has assigned to the variable DESNE5. If the desiredengine speed is greater than maximum speed, then an overspeed conditionexists and the desired engine speed is redefined to be equal to themaximum speed. Alternately, if the pump displacement has been less than5% for more than two seconds, then the variable DESNE5 has been set tostandby speed. If, at this time, the desired engine speed is greaterthan the standby speed, then desired engine speed is redefined to be thestandby speed.

The variable DESNE is delivered to a filter 214 to prevent suddenchanges in engine speed. For example, with the engine running at maximumspeed, should the operator discontinue an operation which requires highoutput then desired engine speed will change somewhat drastically in avery short period, giving the appearance of jerky operation. The filter214 causes the desired engine speed to change more slowly and operationappears much smoother. The desired engine speed is delivered to both thefuel control and underspeed control means 34,36 as discussed earlier.

INDUSTRIAL APPLICABILITY

In the overall operation of the excavator, assume that the operator istrenching, and at this particular portion of the work cycle he ispositioning the bucket to make a cut. The load experienced by thehydraulic implements 56,58 is low to moderate and the hydraulic loadsensing apparatus has positioned the swash plate 30 to provide, forexample, approximately 25% pump displacement. The pump displacement isdetected and the control means 38 sets desired engine speed to theworking speed requested by the operator.

As the bucket begins the cut, the hydraulic load on the implements 56,58increases. The load sensing apparatus 28 responds by increasing pumpdisplacement to approximately 90% to provide the additional flowrequired. A corresponding increase in desired engine speed to 1900 rpmoccurs in response to the increased pump displacement, but the increasedhydraulic load reduces the engine responsiveness, and the actual enginespeed lugs below the desired engine speed. The underspeed control means36 responds by stroking back the pump 30 according to the proportionalplus derivative means 136 shown in FIG. 4. This reduced swash plateposition is detected by the control means 38 which summarily reduces thedesired engine speed signal to correspond to the new swash plate 30position. The fuel control means 34 maintains full rack as long asdesired engine speed is greater than actual engine speed. Thus, theengine is accelerating under the reduced load and the underspeed controlmeans 36 increases swash plate position as the difference betweendesired and actual speed diminishes. However, as the swash plateposition increases, so too does the desired engine speed signal.Judicious selection of the gains K_(P2) , K_(D2) allow the control means38 and the underspeed control means 36 to interact when operating on theproportional portion of the curve shown in FIG. 7, and provide thedesired relationship between engine speed and pump displacement.

At the end of the cut, hydraulic load decreases, the load sensingapparatus reduces displacement, desired engine speed is reduced, and thefuel control means responds to actual engine speed being greater thandesired engine speed by reducing rack displacement until desired equalsactual engine speed. The first proportional plus derivative feedbackmeans forces the fuel control means to reduce the rack position less asthe difference between actual and desired engine speed becomes less atan increasing rate.

At any point during the work cycle, should the operator pause and allowthe load sensing apparatus to stroke the pump displacement to less than5% for longer than two seconds, then the control means 38 will setdesired speed to the standby speed of about 1140 rpm. The fuel controlmeans will reduce rack position and force the engine to slow actualspeed to the targeted standby speed.

While the present invention has been described primarily in associationwith hydraulic excavators, it is recognized that the invention could beimplemented on most any prime mover and hydraulic pump arrangement.

Other aspects, objects, and advantages of this invention can be obtainedfrom a study of the drawings, the disclosure, and the appended claims.

We claim:
 1. An apparatus for controlling an internal combustion enginehaving a rack for controlling a fuel injection pump, and at least onevariable displacement hydraulic pump, said pump having a load sensingmeans for detecting hydraulic load and adjusting inclination of a swashplate in response to hydraulic flow and load requirements, the apparatuscomprising:first means for detecting the displacement of said hydraulicpump and delivering a first signal responsive to the displacement ofsaid hydraulic pump; second means for detecting the rotational speed ofsaid engine and delivering a second signal responsive to said rotationalspeed; control means for receiving said first signal and delivering athird signal responsive to the magnitude of said first signal;underspeed control means for receiving said second and third signals,comparing said second and third signals, and delivering a fourth signalin response to said third signal being greater than said second signal;swash plate actuator means for receiving said fourth signal and reducingthe angle of inclination of said swash plate by an amount responsive tothe magnitude of said fourth signal; fuel control means for receivingsaid second and third signals, comparing said second and third signals,and delivering a fifth signal in response to said third signal beingless than said second signal; and, rack actuator means for receivingsaid fifth signal and controlling the supply of fuel to said engineresponsive to the magnitude of said fifth signal.
 2. The apparatus, asset forth in claim 1, including a plurality of variable displacementhydraulic pumps, each pump having a load sensing apparatus for adjustinginclination of a swash plate of each pump to match hydraulic flow andload requirements, and a plurality of first means for detecting thedisplacement of each of said pumps and delivering first signalsresponsive to the displacement of each of said hydraulic pumps, whereinsaid control means receives said first signals and delivers a desiredengine speed signal responsive to the first signal of greatestmagnitude.
 3. The apparatus, as set forth in claim 1, wherein said swashplate and rack actuator means control pump displacement and quantity offuel injected in response to the duration of said fourth and fifthsignals, respectively.
 4. The apparatus, as set forth in claim 1,wherein said underspeed control means includes a first proportional plusderivative feedback means for controlling the magnitude of said fourthsignal.
 5. The apparatus, as set forth in claim 4, wherein said firstproportional plus derivative feedback means delivers a sixth signal andincludes a first actuator setpoint means for delivering a seventh signalof constant magnitude representative of a maximum angle of inclinationof said swash plate, and means for summing said sixth and seventhsignals and delivering an eighth signal in response to said summationfor controlling the magnitude of said fourth signal.
 6. The apparatus,as set forth in claim 5, including a processing means for receivingeighth signal and accessing a preselected memory location containing abinary number indicative of the magnitude of said fourth signal.
 7. Theapparatus, as set forth in claim 1, wherein said fuel control meansincludes a second proportional plus derivative feedback means forcontrolling the magnitude of said fifth signal.
 8. The apparatus, as setforth in claim 7, wherein said proportional term is zero in response tosaid second signal being less than said desired engine speed signal. 9.The apparatus, as set forth in claim 7, wherein said second proportionalplus derivative feedback means delivers a ninth signal and includes asecond actuator setpoint means for delivering a tenth signal of constantmagnitude representative of a maximum allowable rack position, and meansfor summing said ninth and tenth signals and delivering an eleventhsignal in response to said summation for controlling the magnitude ofsaid fifth signal.
 10. The apparatus, as set forth in claim 9, includinga processing means for receiving said eleventh signal and accessing apreselected memory location containing a binary number indicative of themagnitude of said fifth signal.
 11. The apparatus, as set forth in claim1, wherein said third signal is a desired engine speed signalcontrollably set to one of a plurality of preselected levels in responseto said first signal being within one of a plurality of correspondingranges.
 12. The apparatus, as set forth in claim 11, wherein saiddesired engine speed signal is a first preselected level in response tosaid first signal being less than a first preselected magnitude for apreselected duration of time.
 13. The apparatus, as set forth in claim12, wherein said desired engine speed signal is a second preselectedlevel in response to said first signal being in a range between saidfirst preselected magnitude and a second preselected magnitude.
 14. Theapparatus, as set forth in claim 13, wherein said desired engine speedsignal is directly proportional to said first signal in response to saidfirst signal being greater than said second preselected magnitude. 15.The apparatus, as set forth in claim 13, wherein said second preselectedlevel is adjustable to a plurality of discrete levels.
 16. A method forcontrolling a hydrostatic drive apparatus, the apparatus including aninternal combustion engine controlled by a fuel injection pump actuatorand at least one variable displacement hydraulic pump having a loadsensing means for detecting hydraulic load and adjusting inclination ofa swash plate to match hydraulic flow and load requirements, the methodcomprising the steps of:(a) detecting actual rotational speed of saidengine; (b) delivering a first signal responsive to said actual enginespeed; (c) detecting the displacement of said hydraulic pump; (d)delivering a second signal responsive to said hydraulic pumpdisplacement; (e) receiving said second signal; (f) converting thesecond signal to a desired engine speed; (g) delivering a third signalresponsive to said desired engine speed; (h) receiving said first andthird signals; (i) comparing said first and third signals; (j) reducingthe angle of inclination of said swash plate in response to said thirdsignal being greater than said first signal; and, (k) reducing thesupply of fuel to said engine in response to said first signal beinggreater than said third signal.
 17. The method, as set forth in claim16, wherein step (d) includes determining the magnitude of thedifference between said first and third signals, step (e) includesreducing the angle of inclination of said swash plate by an amountresponsive to the magnitude of said difference, and step (f) includesreducing the supply of fuel to said engine by an amount responsive tothe magnitude of said difference.